DOI: 10.1111/wre.12153 Classification of herbicides according to chemical family for weed resistance management strategies – an update A FOROUZESH*,EZAND*,SSOUFIZADEH† & S SAMADI FOROUSHANI* *Iranian Research Institute of Plant Protection, Agricultural Research Education and Extension Organization (AREEO), Tehran, Iran, and †Department of Agroecology, Environmental Sciences Research Institute, Shahid Beheshti University, G.C., P.O. Box 19835-196, Tehran, Iran Received 24 January 2014 Revised version accepted 26 January 2015 Subject Editor: Enrique Barriuso, INRA, France mechanisms of action. According to this study, these Summary moieties have been assigned to the names of chemical There are inaccuracies in the chemical families of the families and active ingredients are then classified WSSA and HRAC herbicide classification systems within the chemical families accordingly. This study which could limit their practical use in herbicide-based has 119 chemical families, compared with 145 in the weed management strategies. In essence, these inaccu- WSSA system and 58 in the HRAC system. A major racies could be divided into four parts: (i) the nomen- priority of this study is the number of active ingredi- clature of many of the chemical families is not correct, ents covered; we included 410 active ingredients with (ii) distinct active ingredients are grouped in same known mechanisms of action and herbicide groups, chemical families, (iii) many chemical families have more than 100 active ingredients more than the current been repeated in at least two modes of action/herbicide classification systems. Overall, this study provides bet- groups, and (iv) many active ingredients have not been ter opportunities for the management of resistance to assigned to chemical family, herbicide group or mode herbicides through the application of improved pure of action. The aim of this study was to revise the cur- and applied knowledge. rent classifications and to propose corrections for the Keywords: herbicide resistance, cross-resistance, chemi- current ones. Detailed investigations on chemical cal structure, active ingredient, mode of action, Herbi- structure of the active ingredients of the registered her- cide Resistance Action Committee. bicides showed that some moieties have the same FOROUZESH A, ZAND E, SOUFIZADEH S&SAMADI FOROUSHANI S (2015). Classification of herbicides according to chemical family for weed resistance management strategies – an update. Weed Research 55, 334–358. numerous studies about the advantages of rotational Introduction application of herbicide groups in delaying the evolu- Weed resistance to herbicides has become a major con- tion of herbicide-resistant weeds (Stephenson et al., cern in crop production worldwide that necessitates 1990; Gill, 1995; Itoh et al., 1999; Singh et al., 1999; special attention. Resistance to herbicides is often Hartmann et al., 2000; Legere et al., 2000; Hashem attributed to lack of rotational programmes of herbi- et al., 2001; Schmidt et al., 2004). Thus, a true under- cides and to continuous applications of herbicides with standing of the sites of action of herbicides is essential the same sites of action (Beckie et al., 2006). There are for strategic planning of herbicide-based weed control. Correspondence: Saeid Soufizadeh, Department of Agroecology, Environmental Sciences Research Institute, Shahid Beheshti University, G.C., P.O. Box 19835-196, Tehran, Iran. Tel: (+98) 21 2990 2867; Fax: (+98) 21 2990 2867; E-mail: s_soufi[email protected] © 2015 European Weed Research Society 55, 334–358 Herbicide classification 335 An effective way to design a good rotational herbi- As a result, the names of some chemical families (as it cide programme would be to use classification systems stems from the active ingredients) are partially or thor- of these chemicals. Currently, there are two systems of oughly revised. Addition of more than 100 active classification for herbicides: one that belongs to the ingredients may also be an important step towards WSSA (2007) and the other that has been developed decreasing the occurrence of herbicide resistance. by the HRAC (2010). The former classification system uses numbers, whilst the latter uses letters for classify- Chemical basis of this study ing herbicides. Generally, the WSSA system includes 200 active ingredients and 145 chemical families and The basis of the new classification system relies on the the HRAC system covers 291 active ingredients fact that it is the active ingredient that determines its grouped in 58 chemical families. However, these two respective chemical family and ultimately the mecha- systems both have problems with the designation of nism of action it demonstrates (or it should be grouped chemical families that have negative implications for in). However, less familiarity with rules that govern chemical management of weeds. In a broad sense, we the nomenclature of organic compounds is the main divide these difficulties into two aspects. From a chem- cause of mistakes that exist in the HRAC and WSSA istry point of view, (i) there are many active ingredi- classifications. Some examples will be given to clarify ents with unknown chemical family, (ii) there are some this. Chemical family aryloxyphenoxypropionate mistakes in the nomenclature of chemical families, and (WSSA group 1 and HRAC group A) exist in both (iii) some active ingredients have been grouped in a classifications. This chemical family includes some chemical family when they are distinct. In the latter aromatic active ingredients, for example clodinafop, fe- case, such an issue may result in incorrect decision- noxaprop, fluazifop, haloxyfop, propaquizafop and making over the selection of herbicides to be used in a quizalofop. However, the correct name should be hy- rotational programme. As an example, if two distinct droxyphenoxyisopropionic acid. The justification is active ingredients are grouped in the same chemical that according to IUPAC (1979), the term ‘aryl’ refers family, it may be incorrectly inferred that as these two to a univalent aromatic hydrocarbon that only con- active ingredients are members of a same chemical tains carbon and hydrogen atoms. However, in the family, they will show the same cross-resistances. From current classification systems, the term ‘aryl’ has been a weed management point of view, there are 100 active used for aromatic compounds that also contain other ingredients that have not been assigned to any mecha- chemical elements other than carbon and hydrogen, nism of action, chemical family and herbicide group in such as nitrogen; using the term ‘aryl’ for a heterocy- both classification systems. This poses difficulties in clic aromatic compound is not correct. Also, according weed management programmes and could potentially to the WSSA and HRAC systems, the chemical family accelerate the occurrence of resistance. of the active ingredients clethodim and tralkoxydim Considering the shortcomings stated above, this from acetyl CoA carboxylase (ACCase) inhibitors is study aimed at revising the current classification sys- cyclohexanedione. Cyclohexane consists of a six-mem- tems, with the ultimate goals of correcting the record bered ring with six carbon atoms that are connected to and in supporting better planning of weed manage- each other with six single bonds, whilst such a chemi- ment strategies regarding rotational applications of cal structure belongs to cyclohexene. The difference herbicides. The objectives of this study can be summar- between cyclohexene and cyclohexane relates to the ised as follows: (i) corrections to be made in the replacement of a single bond by a double bond in the nomenclature of chemical families based on the rules ring of the latter. The suffix ‘-dione’ in cyclohexanedi- that govern organic chemistry, (ii) distinct active ingre- one indicates the replacement of two >CH2 groups by dients that have been grouped in one chemical family >CO in the ring, whilst in the active ingredients of this to be shifted to appropriate families, (iii) active ingre- chemical family, one >CH2 group is replaced by >CO dients with unknown chemical family and many active in the ring. Thus, the nomenclature of this chemical ingredients repeated with at least two mechanisms of family should be hydroxyoxocyclohexenecarbaldehyde action/herbicide groups to be assigned to appropriate oxime. As another example, 32 active ingredients chemical family and (iv) over 100 active ingredients within the photosystem II inhibitors have the moiety with known mechanisms of action, herbicide groups (moiety is a part of a molecule that may include either and chemical families to be added to both classification whole functional groups or parts of functional groups systems. The authors note that a mechanism of action as substructures; a functional group has similar chemi- directly depends on its active ingredient; thus, the cal properties whenever it occurs in different com- mechanism of action of most active ingredients can be pounds) triazinediamine and thus their chemical family identified merely on the basis of the chemical structure. in this study is named triazinediamine. Within the © 2015 European Weed Research Society 55, 334–358 336 A Forouzesh et al. Table 1 The numbers of mechanisms of action, herbicide groups, chemical families and active ingredients in each classification system Classification systems Mechanisms of action HRAC groups WSSA groups Chemical families Active ingredients This study 16 23 28 119 430 HRAC (2010) 16 23 26 58 291 WSSA (2007) 16 22 28